Apparatus for oxygen determination

ABSTRACT

A DEVICE FOR MEASURING OXYGEN CONTENT OF FLUIDS AT ELEVATED TEMPERATURE COMPRISES A GALVANIC CELL WITH A SOLID OXIDE ELECTROLYTE AND A REFERENCE ELECTRODE OF A MIXTURE OF CHROMIUM OR AN ALLOY THEREOF AND CR2O3. WHEN THE FLUID, ACTING AS THE OTHER ELECTRODE, CONTACTS THE ELECTROLYTE, THE RESULTING EMF INDICATES OXYGEN CONTENT. PROVISION IS   ALSO MADE FOR SIMULTANEOUS TEMPERATURE DETERMINATION AND FOR ELECTRICAL CONTACT WITH FLUIDS, SUCH AS LIQUID STEEL.

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l/V VEN TORS' RICHARD J. F RUEHA/V E' THE M 7T TURKDUGA/V lay R. J.FRUEHAN ET AL AND APPARATUS FOR OXYGEN DETERMINATION TEMPERATURE OXYGENIND/CA TOI? March 27, 1973 Filed May 21, 1970 ,mii/ZW( w75.

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R. J. FRUEHAN ET Al- APPARATUS FOR OXYGEN DETERMINATION Sheets-Sheet 2RICHARD J. FRUEHAN THEM T. TURKDOGAN Homey United States Patent O3,723,279 APPARATUS FOR OXYGEN DETERMINATION Richard J. Fruehan,Franklin Township, Westmoreland County, and Ethem T. Turkdogan,Pittsburgh, Pa., assignors to United States Steel CorporationContinuation-impart of application Ser. No. 727,314, May 7, 1968. Thisapplication May 21, 1970, Ser.

Int. cl. Goin 27/46 U.S. Cl. 204-195 S 10 Claims ABSTRACT OF THEDISCLOSURE This application which is a continuation-in-part of ourco-pending application, Ser. No. 727,314, iiled May 7, 1968, nowabandoned, relates to apparatus for measuring the oxygen content and/oractivity of fluids at elevated temperatures, and more particularly, toapparatus that rapidly determines the oxygen content of liquid steel ina ladle or furnace without sampling. Since our invention, at present, ismost useful and most needed for this purpose, this use Will be stressedhereinafter. However, our invention is also applicable for determiningoxygen in other fluids at temperatures above approximately 700 C., suchas liquid copper and hot furnace gases.

The steel-making process can be considerably advanced if the amount ofoxygen dissolved in molten steel is continuously and immediately knownduring the various stages of the process. Such knowledge would permitpreviously impossible manipulations of a melt of steel by providingbetter control of the oxidation and deoxidation reactions.

The oxygen content of liquid steel has heretofore been determined by theanalysis of samples of the liquid steel. The various analytical methodsnow used require elaborate equipment and do not provide results rapidlyenough for best control of the steel-making process. A solid oxideelectrolyte galvanic cell with a gaseous reference electrode has beenused in laboratory experiments to provide a rapid determination ofoxygen in liquid steels, but these cells are not practical for use insteel production for various reasons. They are relatively expensive,their accuracy is not satisfactory, and their quality is not uniform norsatisfactory.

According to our invention, we provide an oxygen cell having a solidoxide electrolyte fused in a refractory tube. A thermocouple is embeddedinto a condensed phase reference electrode which is placed inside thetube. The thermocouple, in addition to providing a temperaturedetermination, serves as one connection to an oxygen indieatingapparatus. A dise of molybdenum cermet which contacts the liquid steelis also connected to the oxygen indicating apparatus by a -wire of thesame material that connects the thermocouple to the oxygen indicatingapparatus. A metallic cap, which dissolves in the liquid steel, protectsthe cell or probe from the slag when plunged into the melt. Within a fewseconds after immersion in the liquid steel, a steady and reliable EMFis achieved which is measured by the oxygen indicating apparatus.

It is therefore an object of our invention to provide apparatus forrapid or continuons determination of the oxygen content in hightemperature uid over a wide range of temperatures.

Patented Mar. 27, 1973 Another object is to provide a disposable probefor determining oxygen content in molten metal `without sampling themelt.

A further object is to provide such a probe which iS relativelyinexpensive, reliable and accurate.

A still further object is to provide such a probe that requires nothermo-electromotive force corrections.

Still another object is to provide such a probe which also determinesthe temperature of the liquid metal.

These and other objects will be more apparent after referring to thefollowing specification and attached draw ings in which:

FIG. 1 is a sectional view of the oxygen probe of our invention andincludes its connections to be oxygen and temperature indicatingapparatus;

FIG. 2 is a graph showing the relationship between the measured EMF andoxygen content in parts per million; an

FIG. 3 is a graph comparing the measured oxygen content of liquid steelwith the oxygen content determined by the neutron activation method ofanalysis.

Referring more particularly to the drawings, reference numeral 2indicates a calcia stabilized zirconia,

Zr02(CaO) disc `which is placed in one end of a refractory container 4.Disc 2 may be fused to the bottom of a silica tube 4, or inserted intothe bottom of an alumina tube 4. Disc 2 is preferably about 3 mm. indiameter and 3 mm. high. Contained in tube 4 above disc 2 is a mixture 6of Cr and Cr2O3, about 90 to 98 weight percent Cr, the balance Cr203.Partially embedded in the mixture 6 is a conventional thermocouple 8having a double bore silica tube 10 and a pair of wires 12, preferablyPt-Pt/ 10% Rh in contact with the mixture 6. The upper ends of wires 12are each connected to Contact prongs 14. Tube 4, thermocouple 8 and partof prongs 14 are enclosed in an alumina container tube 16. A disc 18 ofmolybdenum cermet, approximately to 85 weight percent Mo and the balanceA1203, is fitted tightly into the bottom of a silica tube 20 to preventthe penetration of liquid metal into the inside of tube 20. A Pt Wire 22is attached to disc 18 and to a contact prong 24 through a silicaprotective tube 26. Tube 16 is filled with a refractory cement 28, suchas Alundum, which is an electrical and thermal insulator. Tube 20 isfilled with alumina powder 29. 'The top of tube 16 is covered with a cap30 made of a plastic, such as Teflon. Prongs 14 and 24 project throughcap 30 to form an electrical plug. An alumina support ring 32 isattached to the bottom of tube 16. The bottom of tube 16 is sealed by ametal cap 34. A long protective cardboard tube 36, only a portion ofwhich is shown, rests on support rings 32. A receptacle 3S fits overprongs 14 and 24 with lead wires 40 connecting prongs 14 and 24 to atemperature and oxygen indicator 42. Prongs 14 and their respective leadwires 40 are compensating alloys conventional for a Pt-Pt/ 10% Rhthermocouple. Prong 24 and its lead Wire 40 is the same compensatingalloy as that connected to the Pt thermocouple wire. Indicator 42 ispreferably a potentiometric recorder or similar device with t-woindicating arms. One indicating arm, for indicating temperature, isconnected to wires 12. A second indieating arm, indicating in the rangeof minus 0.2 volt to plus 0.5 volt, is for oxygen content, and isconnected to Wire 22 and Pt wire 12.

In operation, the probe is plugged into a permanent receptacle providedat the furnace or ladle above the liquid metal L and then plunged intothe melt to a suitable depth. The cardboard tube 36, which extends abovethe top of the melt, protects the wires and upper elements of the probefrom damage by the metal. lf the protective cardboard tube causessignificant local carbon deoxidation, this may be overcome by replacingthe last few inches of the cardboard tube with a refractory tube, suchas silica. Cap 34 protects the probe from chemical attack by the slag asthe probe is lowered through the slag into the molten metal. The cap 34is made of material, such as copper or brass, which will not affect theoxygen content of the steel and which readily dissolves in the steel soas to expose the electrolyte and the molybdenum cermet disc to themolten steel. The cap 34 may also be made of steel. When the stabilizedzirconium oxide electrolyte contacts the electrode of liquid steel, agalvanic cell results which may be represented as follows:

Pt(s), Cr(s)+Cr203(s)[Zr (CaO) (s)|Fe-O (1),

(Mo cermet), Pt(s) The voltage across the electrolyte is given by thefree energy equation as follows:

ercntO glp e l RT E=2.303 -10 KP02,

nF (l) where The temperature dependence of the equilibrium constant Kand 1102i for Cr-Cr203 equilibrium from known thermodynamic data is19,700 10g @021/2 (atl'ILl/z) By combining these equations andsubstituting values of the constants, the following equation is obtainedfor weight percent oxygen in liquid steel in terms of EMF, (E) inmilivolts and T in degrees Kelvin for the Cr-Cr203 reference electrodeIt should be noted that E is actually a measure of the oxygen activityin liquid steel. The relationship of the activity of the oxygen in theliquid steel to the oxygen content of the liquid steel is well known andis shown in FIG. 2 where the EMF indicates the oxygen activity.

By using lead wires to the oxygen indicator of the same material,preferably platinum, no thermo-electromotive force corrections arerequired since all connections are at the liquid metal temperature andthe lead wires will be at the same temperature. Fusing a small disc ofelectrolyte into the silica tube greatly increases its resistance tothermal shock.

While the EMF of Equation 5 neglects the effect of alloying elements insteel, the equation is essentially correct for low alloy steels. Anapproximate correction for alloying elements can be made by thefollowing expression:

Actual log (wt. percent O) :Apparent log (percent O) (e, percent -l-ejpercent j -I-ek percent k-llog (percent) 0=4.62

where apparent log percent 0=the value from Equation 5 using measured Epercent i, percent j, percent k, etc.==concentrations of variousalloying elements in liquid steel, such as Mn, Si, C, etc.

ei, ej, ek, etc.=interaction coefficients for various alloying elements.

For large tonnage, low carbon steel-making the concentrations of Mn, Si,C are smal lenough that the above correction becomes insignificant forall practical purposes.

A steady EMF is achieved in a few seconds after plunging the probe intothe melt and provides continuous oxygen content readings up to 40minutes. The cell has sufficient reliability and accuracy for use in thesteelmaking process as shown in FIG. 3 where the oxygen content ofsamples determined by the neutron activation method is compared with theoxygen content as measured by the EMF of the cell.

While the specific components set forth above are preferred since theyhave proven very successful in use in determining the oxygen content ofliquid steel, they may be varied as long as they meet the followingrequirements. The electrolyte 2 must be a solid oxide and an oxygen ionconductor with insignificant electronic conductivity under theconditions of its use, particularly at the ternperature and oxygenpartial pressure to which it is subjected. It must not react with thematerials it contacts. Zr02 with 3 to 10% by weight of Ca0 and Th02 with3 to 20% by weight of Y203 are preferred for use with steel. MgO, A1203,and Zr02 with 3 to 10% by weight of MgO or Y2O3 and Th02 with La203 mayalso be used, but electrolytes are not all suitable for use with alltypes of fluids.

The reference electrode y6 must be a condensed phase mixture of chromiumand its oxide which does not readily melt at the temperature to which itis subjected nor react significantly with the materials it contacts. Itis preferred to use a mixture of between about 90% to 98% by weight ofpure chromium and the remainder Cr203. However, a chrome alloy may besubstituted for the chromium. The alloying metal must be one having anoxide less stable than Cr203 so that Cr203 will be the equilibrium oxidephase. The melting point of the alloy must be high enough so that thealloy is solid at temperature of use. The dissociation pressure inEquation l would be corrected accordingly. Suitable alloys include up to10% by weight of manganese or iron, or up to 5% nickel.

While it is preferred to use a Mo cermet Mo, 20% A1203) for member '118,it is only necessary that the material be an electronic conductor, willnot melt at its temperature of use, will not readily dissolve in thefluid so that a steady and reliable voltage reading can be taken, andwhen dissolved in the liquid will not affect the voltage reading. Suchmaterials are Cr cermet (80% Cr, 20% Al203), Cr203, Mo wire or rod, Fewire or rod and graphite rod. Not all materials can be used for alltypes of liquids.

A probe suitable for determining the oxygen content of liquid copper mayhave a reference electrode of Cr-Cr203, and a liquid metal contact of Crcermet or Cr203.

When used for determining the oxygen contents of the gases in soakingpits, heat treatment furnaces, hot exhaust gases or the like, the probeis altered slightly. Wire 22 must contact the electrolyte 2 where it isin contact with the gas and the cermet contact may be omitted. Thebalance of the probe is, of course, not in contact with the gas.

The specific probe described has been used in the oxygen content rangefrom about 10 p.p.m. to about 1000 p.p.m. Below that concentration it ispreferred to use ThO2(Y2O3), which will provide predominantly ionic conduction at oxygen contents down to 1 p.p.m.

While silica has been used extensively in the probe, other refractories,such as alumina, may be used which would better resist the effects ofliquid steel. The refractory must, of course, not react with othercomponents of the probe.

Since the probe is made of inexpensive materials and is relatively easyto manufacture, the probe can be used once and thrown away in the samemanner that immersion thermocouples are used. Because a conventionalthermocouple is incorporated into the probe, temperatures may bedetermined at the same time as oxygen content, and auxiliary equipmentrequirements are minimal.

While the preferred embodiment of our invention is described as aninstrument to use in a vessel, ladle or furnace as a conventionalimmersion thermocouple is used, other adaptations are possible. As anexample, a zirconia thimble, small enough to avoid thermal shock, couldbe located in the lining of a tundish to contact the liquid steel withthe other essential parts of the probe arranged in a manner alreadydescribed.

While several embodiments of our invention have been shown anddescribed, it will be apparent that other adaptations and modiiicationsmay be made 'without departing from the scope of the following claims.

We claim:

1. Apparatus for determining the oxygen content of a fluid at atemperature of at least 700 C. which comprises a galvanic cell includinga refractory container closed at one end thereof by a solid oxideelectrolyte adapted to contact said fluid, and a reference electrodeinside said refractory containers and in electrical contact with saidelectrolyte, said electrolyte being an oxygen ion conductor havinginsignificant electronic conductivity under conditions of use, saidreference electrode being a mixture of Cr203 and a material of the classconsisting of chromium and an alloy of chromium with a metal having anoxide less stable than Cr203, said material having a melting point suchthat it will be solid at the temperature of said fluid, and means formeasuring the EMF of the cell when the electrolyte contacts the uid.

2. Apparatus according to claim 1 in which said electrolyte is of thegroup consisting of MgO, A1203, Zr02 with 3 to 10% by weight of CaO,ThO2 with 3 to 20% by weight of Y2O3, ZrOZ with 3 to 10% by weight ofMgO, Zr02 with 3 to 10% by weight of Y2O3, and 'I'h02 with La203.

3. Apparatus according to claim 1 which includes a refractory electronicconductor adapted for electrical contact with said electrolyte, saidconductor being made of material which is relatively insoluble in saiduid, and means connecting said conductor and said reference electrode tosaid EMF measuring means.

4. Apparatus according to claim 3 in which said conductor is of thegroup consisting of a molybdenum cermet composed of about 80% by weightof Mo and about 20% by weight of A1203, a chromium cermet composed ofabout 80% by weight Cr and about 20% by weight of A1203, molybdenum,graphite and Cr203.

5. Apparatus according to claim 4 in which said EMF measuring means is atwo position potentiometric recorder; and which apparatus includes athermocouple, said thermocouple having a junction embedded in saidreference electrode, a first thermocouple wire connected to bothpotentiometric positions, and a second thermocouple wire connected toone potentiometric position to indicate temperature; and in which saidmeans connecting said conductor to said EMF measuring means includes ajunction of said conductor and a wire of the same material as vsaidfirst thermocouple wire, said last named wire being connected to theother potentiometric position to indicate oxygen content, and saidjunctions being located to operate at the same temperatures underconditions of use.

6. Apparatus according to claim 1 in which said electrode is a mixtureof between about to 98% by weight of chromium and the remainder Cr203.

7. Apparatus according to claim 6 for use in a melt of steel having alayer of slag thereon, which apparatus includes a steel-soluble capshielding said apparatus from slag upon immersion with said melt, saidcap being of the group consisting of copper, brass, and steel.

8. Apparatus according to claim 7 which includes a refractory electronicconductor adapted to contact said melt, said conductor being made ofmaterial which is relatively insoluble in said melt, and meansconnecting said conductor and said reference electrode to said EMFmeasuring means.

9. Apparatus according to claim 1 in which said material of saidreference electrode is an alloy of chromium with another metal.

10. Apparatus according to claim 9 in which said alloying metal is ofthe group consisting of up to 10% by weight manganese, up to 10% byweight iron and up to 5% by weight nickel.

References Cited UNITED STATES PATENTS 3,464,008 8/1969 Meysson et al204-195 3,468,780 9/ 1969 Fischer 204-195 3,481,855 12/1969 Kolodney etal 204-195 3,619,381 11/1971 Fitterer 204--195 S OTHER REFERENCESFitterer: Reprint from Journal of Metals, August 1966, pp. 1-6.

Tretjakow et al.: Berichte der Bunsengesellschaft, vol. 69, No. 5, 1965,pp. 396-402.

Horsley: A ERE Report R3427, pp. l8 and FIG. 2, 1961.

TA-HSUNG TUNG, Primary Examiner U.S. C1. X.R. 204-1 T Patent No.3v723v279 Dated MaC' 27 1973 Inventms) RICHARD J., FRUE E'ua AL.

It is certified that` error appears in the above-dentfied patent andthat said Letters Patent are hereby correclted as shown below:

Col 5p line 28 mwont-,aimersW should fread M- container msigned andvsealed this 20th day of Noven'ber 1973.,

(SER1.)

Attest:

Attesting Officer Acting Commissioner of Patents FORM PO-105O (1C-59)USCOMMDC 50376-P69 U.S. GOVERNMENT PRINTING OFFICE |969 0-'366-3342;

